1. Field of the Invention (Technical Field)
The present invention relates to the control of Electro-Slag Remelting (ESR) furnaces, which are used to process metals and alloys to improve material quality. An electrode of the material to be processed is melted by the ESR process to produce a refined ingot. The process is characterized by the passage of current between the electrode and ingot through a molten, ionized slag. This slag is sufficiently heated by passage of current (Joule heating) so that the electrode is melted as it is immersed in the slag. The molten metal drips from the electrode through the slag and forms an ingot below.
2. Background Art
The ESR process is traditionally carried out commercially with alternating current (AC), and most control processors rely on RMS (root mean square) values of voltage and current to provide process information, while others gather information on the resistivity or conductivity of the AC circuit. Control of an ESR furnace is commonly achieved by maintaining a constant current and regulating the voltage between the electrode and the ingot. The electrode is servoed in an attempt to keep voltage constant, and a swing controller is employed to adjust the voltage that the ram servo system tries to maintain to keep the magnitude of oscillation (and thereby the immersion depth) constant. However, the control strategies currently used in industry do not provide for accurate control of electrode immersion depth, resulting in imperfections in the ingot being produced.
The difficulties in controlling ESR result largely from the fact that control systems relying on constant current operation are inherently unstable. The present invention is of a control system that is inherently stable due to employment of a substantially constant voltage rather than permitting voltage to change while holding current constant.